Currently, LED manufacturing yields are quite heavily affected by variations in characteristics relating to the change in their luminance with applied voltage L(V) or the variation in their luminance with control current L(I), or indeed in the maximum luminance that may be obtained Lmax.
In general, lamp or bulb manufacturers mount tens, or indeed hundreds, of individual LEDs in a system. These LEDs are connected either in series (the same current for all) or in parallel (the same voltage for all) or else by combining the two. Variations from LED to LED have a very negative effect on the uniformity of appearance, indeed for the functionality of the system, resulting in integrators having to specify an acceptable range of pairs in terms of control voltage and luminance (V, L) or of control current and luminance (I, L). LED manufacturers must therefore sort components on the basis of these parameters at the end of the production line and reject all those components which do not fall within the desired range. The greater the dispersion in the performance of components, the greater the drops in yield.
In general, to produce an LED device, individual electroluminescent elements, which may be 2D or 3D at the emission surface, can be formed on the surface of one and the same substrate, each of these individual elements, which are connected in series and/or in parallel, being supplied with power so as to produce light, via a control circuit. It is still possible to have variations in performance from one individual element to the other, resulting in fluctuations in overall performance levels, while integrators have to meet performance constraints that fall within a defined range.
Generically, it is specified that an LED chip corresponds to a monolithic element the matter of which has been organized so as to form an electroluminescent structure, having at least two electrical accesses for its power supply. An LED component is a chip (monolithic element) in a package that provides all or some of the electrical, thermal, mechanical and optical interfaces with the user system. It should be noted that the package may amount to a particular coating of the chip allowing it to be directly used in the system (the chip-on-board technique for example).
More specifically and in a known manner, individual LEDs may feature planar microstructures, referred to as 2D microstructures, and/or 3D microstructures or nanostructures comprising filamentous, conical, frustoconical or pyramidal three-dimensional elements. Throughout the rest of the present description, the term “wire” will refer to any three-dimensional element of one of the above-mentioned types. The lateral dimensions (diameter) of wires may for example be of the order of several hundreds of nanometers and their vertical dimension may reach up to a few tens of micrometers, with a height/diameter ratio of between 1 and 30 and typically of around 10.
In the last few years, visible light-emitting diodes based on vertical InGaN/GaN wires containing a p-n junction and connected collectively in parallel have for example been produced.
By virtue of their potential intrinsic properties (high crystal quality, strain relaxation at the vertical free surfaces, high light extraction efficiency via waveguiding, etc.), wires are also considered to be very promising candidates for the production of electroluminescent devices.
Two approaches for producing LEDs based on wires, using different growth techniques, have already been proposed.
The first technological approach consists in growing GaN wires containing InGaN quantum wells epitaxially in an axial configuration by molecular beam epitaxy (MBE). Devices fabricated from these wires have yielded very promising results in the green spectral domain. Processed chips of 1 mm2 are able to emit about 10 μW at 550 nm for a DC operating current of 100 mA.
With the molecular beam epitaxy (MBE) growth technique, certain non-uniformities appear because of random nucleation mechanisms, but typically an optical power of 50 nW has been obtained for a single wire emitting at 550 nm, i.e. 5 W/mm2 with around a hundred thousand emitting wires/mm2.
More recently, the metal organic chemical vapor deposition (MOCVD) growth technique has allowed InGaN/GaN wires containing a radial LED structure (core/shell configuration) to be produced.
FIG. 1 illustrates this type of configuration, in which wires NT; are produced on the surface of a substrate 11 covered with a nucleation layer 21, a lower contact layer 10 also being provided. Localized epitaxy is achieved by means of a mask 20. The wires have a core/shell structure. The core 30 can comprise an n-doped GaN material, typically with a dopant density of 1019 cm−3; a quantum well structure made up of alternating layers that may potentially be made of InGaN and undoped GaN, respectively; and lastly a shell 31 that can consist of a p-doped GaN layer, typically with a dopant density of 1019 cm−3.
A dielectric layer 40 provides insulation between the lower and upper contacts.
The upper contact is made via a conductive upper layer 50 that is transparent to the emission wavelength of the photoconductive structure. A metal contact layer 60 is also provided.
In this approach, since the LED structure has a core/shell configuration, the area of the active zone is larger than in the 2D wire LED approach.
This property has two advantages: it increases the emissive area and decreases current densities in the active zone. Complete MOCVD wire LED structures have been produced on a silicon substrate, and light emission in the blue spectral domain (450 nm) has been obtained for an integrated array of wires after technological processing.
By virtue of the technologies used to grow the wires, hundreds of thousands of wires may be produced on the surface of a chip on an area potentially, and typically, of 1 mm2.